The standard unit of mass at the atomic level is the unified atomic mass unit (u) or Dalton (Da). This unit is defined based on the mass of a specific isotope of carbon, carbon-12 (^12C), which is assigned a mass of exactly 12 atomic mass units.
The unified atomic mass unit is a unitless quantity that provides a relative measure of mass at the atomic and molecular scale. It allows for convenient comparison of the masses of different atoms and molecules without the need for cumbersome conversion factors.
It's important to note that the unified atomic mass unit is not equivalent to any specific unit of mass in the International System of Units (SI). However, it is related to the SI unit of mass, the kilogram (kg), by the Avogadro constant (6.02214076 × 10^23 mol^-1). One mole of any substance contains Avogadro's number of particles (atoms, molecules, ions, etc.), and its mass in grams is numerically equal to the molar mass expressed in unified atomic mass units.